1. Field of the Invention
The present invention relates to an optical receiver applied to the wavelength division multiplexing (WDM) system and a method to control the optical receiver.
2. Related Prior Art
Recent optical communication system sometimes requests the transmission speed thereof reaching 100 Gbps to solve the bottleneck of the transmission capacity between the servers. One solution for the request has been proposed, in which the system includes an optical transmitter transmits signal light containing four optical signals each emitted from, for instance, an electro-absorption modulated laser (EML) or a direct modulated laser (DML), modulated at about 26 Gbps and multiplexed by an wavelength division multiplexer; a single mode fiber (SMF) with a length of 10 to 40 km to transmit the signal light; and an optical receiver including four receiver modules each receiving optical signals de-multiplexed by an optical de-multiplexer. This system may realize an equivalent transmission capacity of about 100 Gbps.
In such a communication system, because of the large optical loss in the SMF with the 40 km length, the optical receiver has been used to accompany with a semiconductor optical amplifier (hereafter denoted as SOA) in the upstream side of the optical de-multiplexer to compensate the optical loss in the SMF. One technique has been reported to compensate the optical loss dynamically, in which the bias current supplied to the SOA is dynamically adjusted to vary the optical gain of the amplifier based on the input level to the SOA. When the optical input level reduces, the optical receiver increases the bias current to increase the optical gain. While, the receiver decreases the bias current to lower the optical gain of the SOA when the optical input level increases, or flows the bias current in the opposite direction to operate the SOA as a device to absorb light, that is, an optical attenuator.
Another method to prevent the receiver module connected in the downstream side of the optical de-multiplexer from receiving an excessive optical signal has been proposed in a Japanese Patent application published as JP-2004-120669A. The optical receiver disclosed therein provides an optical attenuator in the front end to adjust the optical output level from the SOA. Another arrangement has been disclosed in a Japanese Patent published as JP-2005-064051A where the optical receiver has a function combining the front end attenuator and the variable gain optical amplifier.
In such conventional optical receivers, the optical receiver monitors the optical input level at the upstream side of the SOA, and the bias current supplied to the SOA is so adjusted based on the monitored result, which is inevitable to provide additional optical components such as, for instance, an optical splitter and an optical attenuator, which makes the optical receiver complicated and large-size. The present invention is to provide an optical receiver with a simplified arrangement and a function to prevent the receiver modules from receiving an excessive optical input.